1. Field of the Invention
This invention relates to the cogeneration of space/water heat and electrical power.
2. Background Information
In most regions of North America, space heating of buildings is a necessity for some portion of the year to maintain thermal comfort. Buildings are space-heated in a variety of ways, with one of them being so-called forced hot water or “hydronic” heating, using a pump to recirculate water in a closed system through a “boiler” for heating, through pipes, and to a variety of space heating devices such as baseboard fin-tube radiators, radiant panels, and radiant floor heating tubing or to a number of separately controlled zones incorporating such a variety of space heating devices. The majority of such boilers are warmed directly by using the hot gases produced by a fossil fuel burner. The closed loop hydronic water circuit can also be used to produce a heated or “hot” domestic water supply by way of a heat exchanger.
Buildings also require a source of electric power. Hydronic heating systems are in themselves a user of electric power to operate the hydronic water pump(s), the burner, and the electrically operated valves. This electric power is normally provided by an electric utility through the local electric grid, with the generation of the electric power occurring at large power stations remotely located from the building. All forms of electric power generation at large remotely located generating plants result in a large fraction of the fuel energy being normally dissipated as waste heat. The combination of electric power generation with useful application of the heat energy that is inevitably produced during electric power generation is generally termed “cogeneration.” Cogeneration is the simultaneous production of useful electric power and heat from the same fuel and fuel burner.
Small-scale cogeneration of heat and electric power from fossil fuels to meet the on-site energy needs of residential and commercial buildings represents a major opportunity for reduction of energy costs and pollutant emissions, including CO2, a “so-called” greenhouse gas. There is a general trend in the regulatory management of energy resources to specifically allow and encourage the tie-in of small-scale cogeneration and renewable energy systems into the existing electric utility grid. This benefits the power generating authorities by allowing them to delay construction of new capacity. However, there is as yet no widespread use of small-scale cogeneration. The technical and economic inadequacies of existing small-scale cogeneration technologies, as well as historical energy supply and regulatory practices, have perpetuated this situation. A number of small-scale power generation technologies are emerging that may be used in such small-scale combined heat and power systems. These include internal combustion engines, Stirling engines, fuel cells, and steam engines. Small-scale combined heat and power systems are now commonly referred to as micro-combined heat and power systems or, more briefly, “micro-CHP” systems and will be referred to as such in this discussion for convenience.
To date, little attention has been paid specifically as to how such small-scale power generation technologies could be practically integrated into hydronic (i.e. forced hot water) system. Nearly all candidate generator technologies suitable for use in small-scale cogeneration of electric power and heat incorporate a liquid cooling (for example, glycol, water, and mixtures thereof) circuit. However, there are important practical technical considerations and limitations with respect to use and integration of such candidate electric generator technologies so as to limit the simple attachment of such generators to hydronic heating systems. New integrated design and operation strategies for hydronic heating systems incorporating micro-CHP are needed to gain full performance potential. An integrated system design as described herein that addresses the combined and complementary mechanical, thermal, electrical power and control characteristics of all system components is essential to practical realization of hydronic systems with cogeneration capability.
Many prior art cogeneration systems are targeted toward large-scale facilities, with designs that do not scale-down to a residential/small commercial application. They may involve the use of gas turbines and steam plants that cannot be reproduced for a residence. While attempts to produce a small-scale cogeneration system have been made, these either do not interface with commonly used residential and small commercial hydronic heating systems, or are impractical to employ in a “real-world” application at the residential and small commercial level. In particular, the specific issues associated with operation with a multi-zone residential and commercial hydronic heating systems have not been addressed. To date, micro-CHP system of the size most suitable for residential homes has been significantly commercially introduced only in Japan and the designs of these systems are not generally preferred for North American residential heating applications and customary practices. For example, U.S. Pat. No. 6,435,420 shows one such system originating in Japan. This patent teaches that the system is configured so that heat from the engine unit is transferred, with a separate coolant circuit, into potable water in tank; and the heat is again transferred from the potable water, through another heat exchanger, into a closed hydronic heating loop. This system allows for the use of heated potable water produced by the engine as a reserve of heat energy for hydronic space heating. This system works well for the particular implementation in typical Japanese residence, where the space heating load is low. This system also works effectively where relatively low hydronic heating water temperatures (e.g. potable water temperatures) are permissible, but this system of limited advantage in typical North American applications that tend to require relatively high hydronic water temperatures. Also, this prior art system requires storage tank and heat exchanger assemblies not commonly produced in North America. In addition the above-described prior art design does also not take into account the “net metering” utility interconnection that is available in the United States, and that provides for maximizing the economic advantages of cogeneration under the prevailing operational situation.
Taken individually, or as a whole, the prior art fails (for typical North American thermal energy load characteristics, engineering practices, and commonly-available and cost-effective component configurations) to provide an overall design and operating logic for a practically implemented fuel-burning forced hot water heating system with modern features combined with an efficient, fuel-burning electric generator with liquid heating capability with such combination system providing multi-zone space heating for thermal comfort while simultaneously maximizing the operation of the electric generator for the cogeneration of heat and electric power and providing important additional functionality such as emergency power supply and domestic water heating. Hence, a practical, state-of-the-art and efficient small-scale cogeneration system, that is particularly suitable for use as a modern forced hot water heating system employed in many homes and enterprises, is highly desirable.